I've often asked myself this question...I still think of myself as just a humble beginner. And I suppose one has to keep a sense of proportion: beginner compared to who? intermediate compared to who? I've been playing since '94, but I"m just a beginner compared to some big shots out there.

For me, knowing you've moved to intermediate means you can a) play with a band and hit a nice groove together, and b) your hands don't hurt like after 30 minutes.

Hummmmmmm......how do you know?? ...When you can follow another player..i.e. guitar, keyboards..and know what notes to play for what key they are in..Knowing many songs..and adding to that list all the time..
Taking good care of your gear, always..Knowing when and when not to play..I am only on my first cup of coffee, so this is all I can come up with right now..

When you here a song on the radio and it flashes in your mind what the bass players doing and within just a few mesuers you got it! and this is the first time you hurd the tune

Don't you just love that?? I love that!! When I can listen to a bass line, go right to the note and play the whole bassline, because I can SEE what he did..

I think your intermediate when you can impro and sound decent.

I don't know. Personally I slotted myself in the intermediate area but I think my definition is different. Apparently, this site has 70 something expert musicians. I don't know if I agree with that. John Coltrane was an expert musician. Alain Caron is an expert musician. Flea is NOT an expert musician. To be an expert I think you have to have complete command over your instrument. You have to be able to play anything you hear in your head instantly, and what you play has to be awesome. An intermediate has to be able to improvise musical ideas and be able to work with them through key changes. Even technical wizards who can blow your mind in E all day can only truly be considered intermediates because there is just so much more in music that they do not yet know.